Carbon-carbon coupling reactions catalyzed by transition metal catalysts are among the most important reactions of synthetic organic chemistry. Metathesis of a terminal olefin with itself produces ethylene and an internal olefin (Equation 1):
wherein x is a value between 0 and 2. This is called a homo-metathesis or homo-coupling reaction. Metathesis catalysts for coupling reactions have been described for decades. However, a mixture of the two possible products (E-isomer and Z-isomer) is usually produced, with the E-isomer being the dominant isomer. The Z-isomer, in many cases, is the isomer which is required by organic chemists for the synthesis of pharmaceuticals, or other chemical products. The Z-isomer also is that largely found in natural products that contain an internal 1,2-disubstituted olefin. Mixtures of Z- and E-isomers are usually difficult to separate and are therefore, generally undesirable. Z-selective coupling of internal olefins is much less useful than Z-selective coupling of terminal olefins, since Z internal olefins themselves must be prepared through Z-selective coupling of terminal olefins. Alternative methods of preparing internal olefins (e.g., Wittig chemistry), are generally not catalytic and/or not Z-selective.
Accordingly, improved methods and catalysts are needed.